Threads stuck in zap_pid_ns_processes()

[Guenter Roeck]

Hi all,

the test program attached below almost always results in one of the child
processes being stuck in zap_pid_ns_processes(). When this happens, I can
see from test logs that nr_hashed == 2 and init_pids==1, but there is only
a single thread left in the pid namespace (the one that is stuck).
Traceback from /proc/<pid>/stack is

[<ffffffff811c385e>] zap_pid_ns_processes+0x1ee/0x2a0
[<ffffffff810c1ba4>] do_exit+0x10d4/0x1330
[<ffffffff810c1ee6>] do_group_exit+0x86/0x130
[<ffffffff810d4347>] get_signal+0x367/0x8a0
[<ffffffff81046e73>] do_signal+0x83/0xb90
[<ffffffff81004475>] exit_to_usermode_loop+0x75/0xc0
[<ffffffff810055b6>] syscall_return_slowpath+0xc6/0xd0
[<ffffffff81ced488>] entry_SYSCALL_64_fastpath+0xab/0xad
[<ffffffffffffffff>] 0xffffffffffffffff

After 120 seconds, I get the "hung task" message.

Example from v4.11:

...
[ 3263.379545] INFO: task clone:27910 blocked for more than 120 seconds.
[ 3263.379561]       Not tainted 4.11.0+ #1
[ 3263.379569] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 3263.379577] clone           D    0 27910  27909 0x00000000
[ 3263.379587] Call Trace:
[ 3263.379608]  __schedule+0x677/0xda0
[ 3263.379621]  ? pci_mmcfg_check_reserved+0xc0/0xc0
[ 3263.379634]  ? task_stopped_code+0x70/0x70
[ 3263.379643]  schedule+0x4d/0xd0
[ 3263.379653]  zap_pid_ns_processes+0x1ee/0x2a0
[ 3263.379659]  ? copy_pid_ns+0x4d0/0x4d0
[ 3263.379670]  do_exit+0x10d4/0x1330
...

The problem is seen in all kernels up to v4.11.

Any idea what might be going on and how to fix the problem ?

Thanks,
Guenter

---
This test program was kindly provided by Vovo Yang <vovoy@xxxxxxxxxx>.

Note that the ptrace() call in child1() is not necessary for the problem
to be seen, though it seems to make it a bit more likely.
---

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <errno.h>
#include <string.h>
#include <sched.h>

#define STACK_SIZE 65536

int child1(void* arg);
int child2(void* arg);

int main(int argc, char **argv)
{
  int child_pid;
  char* child_stack = malloc(STACK_SIZE);
  char* stack_top = child_stack + STACK_SIZE;
  char command[256];

  child_pid = clone(&child1, stack_top, CLONE_NEWPID, NULL);
  if (child_pid == -1) {
    printf("parent: clone failed: %s\n", strerror(errno));
    return EXIT_FAILURE;
  }
  printf("parent: child1_pid: %d\n", child_pid);

  sleep(2);
  printf("child state, if it's D (disk sleep), the child process is hung\n");
  sprintf(command, "cat /proc/%d/status | grep State:", child_pid);
  system(command);
  sleep(3600);
  return EXIT_SUCCESS;
}

int child1(void* arg)
{
  int flags = CLONE_FILES | CLONE_FS | CLONE_VM | CLONE_SIGHAND | CLONE_THREAD;
  char* child_stack = malloc(STACK_SIZE);
  char* stack_top = child_stack + STACK_SIZE;
  long ret;

  ret = ptrace(PTRACE_TRACEME, 0, NULL, NULL);
  if (ret == -1) {
    printf("child1: ptrace failed: %s\n", strerror(errno));
    return EXIT_FAILURE;
  }

  ret = clone(&child2, stack_top, flags, NULL);
  if (ret == -1) {
    printf("child1: clone failed: %s\n", strerror(errno));
    return EXIT_FAILURE;
  }
  printf("child1: child2 pid: %ld\n", ret);

  sleep(1);
  printf("child1: end\n");
  return EXIT_SUCCESS;
}

int child2(void* arg)
{
  long ret = ptrace(PTRACE_TRACEME, 0, NULL, NULL);
  if (ret == -1) {
    printf("child2: ptrace failed: %s\n", strerror(errno));
    return EXIT_FAILURE;
  }

  printf("child2: end\n");
  return EXIT_SUCCESS;
}